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Dirt Feedlots and Groundwater |
JUN ZHU
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Many Minnesota dairy farms maintain cows on a combination of pasture and dirt feedlot. Are such feedlots a potential source of contamination for groundwater? A just-completed study suggests that even after 100 years, few pollutants leach more than four feet into the soil. In addition, potential groundwater contamination can be minimized by replacing the top layer of soil every 20 years or so. The research, funded by the Minnesota Department of Agriculture and carried out by the researchers in the University of Minnesota Biosystems and Agricultural Engineering Department and the Southern Research and Outreach Center, investigated 10 randomly selected dairy dirt feedlots in southern Minnesota with ages ranging from 20 to more than 100 years. Seven of the sites were built on loam soil, two were on sandy loam soil, and one was on clay loam soil. Several five-foot-deep soil samples were taken from each feedlot and from the surrounding area. Here's what they found: Soil pH. Soil pH ranged from 7.0 to 8.5. It was usually higher in the top foot than in the second foot. Phosphorus. No significant increases in soil phosphorus were seen more than four feet below ground at any site, regardless of age or soil type. This suggests that dirt feedlots are unlikely to lead to acute phosphorus pollution problems to groundwater as long as the water table is below four feet. Ammonium Nitrogen. For loam soil, ammonium nitrogen concentration was not affected three feet below ground at sites 60 years old or less. The 100-year site showed much higher ammonium nitrogen concentration in all depths. Thus, if the water table is three feet below the surface, loam soil feedlots less than 60 years old may not pose a threat to the groundwater quality, but older feedlots might. Sandy loam soil showed a potential leaching problem for ammonium nitrogen. However, the seepage seemed to be retarded on sites older than 30 years, perhaps due to the clogging of pores and channels between the sand soil particles. In the clay loam feedlot, no increase in ammonium nitrogen was found below three feet. Since only one such site was sampled, however, we need to look at more sites before making any reliable conclusions. Nitrate Nitrogen. In all three soil types, older sites appeared to have a bigger potential for nitrate nitrogen pollution than the younger sites. The biggest increase in nitrate nitrogen concentrations were found in the top foot. Replacing the topsoil layer every 20 years could reduce the buildup of nitrate nitrogen in the topsoil as well as the downward movement of nitrate nitrogen due to saturation of the top layer. In summary, it appears that phosphorus seepage is not a concern as long as the water table is four feet below the surface. Nitrogen seepage has to be considered cautiously on a case-by-case basis. Replacing the top foot of soil every 20 years can minimize the risk of groundwater pollution by ammonium and nitrate nitrogen. |
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